Operation Babel: New Tokyo Legacy

Story related and cannot be missed.

After completing the side mission Space Envoy Contact, which is necessary to continue the story, you will be able to equip a Sub Blood Code. The game makes you select a Sub Blood Code for one character to continue the story, but you can always change it back if you like. I recommend having all six members having a Sub Blood Code equipped anyway.

While having a Sub Blood Code equipped, the main Blood Code will level up more slowly. The XP is distributed in a 2:1 fashion (out of every 3 XP, 2 XP goes to the Blood Code and 1 goes to the Sub Blood Code). Despite this, it's highly recommended to equip Sub Blood Codes not only for their skills, but whenever you level up a Sub Blood Code, you get an HP boost. Blood Codes and Sub Blood Codes can be changed at anytime.